Modular dynamics testers are used to acquire amounts of reservoir fluid for analysis and transportation. The reservoir fluid is drawn into the modular dynamics tester through a probe that is placed in contact with the bore-hole wall. The placement is accomplished by reducing the pressure within the modular dynamics tester tubular that initially contains bore-hole fluid, from the pressure of the formation.
Pressure reduction used for placing the modular dynamics tester tubular is generated by a positive displacement pump operated by hydraulic fluid. In conventional apparatus and methods, piston position is determined with a transducer that is based on the Hall effect. Such conventional apparatus are used within approximately one (1) centimeter of the end cap and provide methods of determining the position as a function of the whole piston movement.
Four (4) effects may be accomplished, including (1) reduction of pressure surges as the piston meets the cylinder end; (2) control of the pump speed to provide pressure control essentially eliminating the reduction of pressure surges as the piston meets the cylinder end; (3) from knowledge of the thermophysical properties of the hydraulic oil, the hydraulic fluid flow rate may be calculated; and (4) from the hydraulic fluid flow rate combined with either calculated or measured thermophysical properties of the reservoir fluid, an estimate may be provided of the flow-rate of the reservoir fluid within the flow-line tubular.
Conventional systems and methods do not accurately determine the position of a piston surface and therefore the inherent problems of pressure surges and improper pump speeds pervade the operations of these systems.